Many parts of automobiles, such as doors and bumpers, or household appliances, such as refrigerator panels, are press-formed from a steel plate or other metal plates. There is an increasing demand for lightweight press-formed products. Therefore, high-strength steel plates are used to provide thin and lightweight products. High-strength steel plates, however, have greater deformation resistance, which may increase the likelihood of occurrence of springback caused by residual stress during the process of press-forming.
There is a recent trend that a forming process planning for forming products is started at the same time as a design stage of automobiles or the like; in order to reduce development man-hours and manufacturing costs. In order to follow the trend, a configuration of a press-formed product and its forming data are analyzed by a computer. The analysis provides calculation of a springback amount of the press-formed product estimated from the residual stress after the forming. The die configuration is corrected in accordance with the calculated springback amount.
Patent Document 1 and Non Patent Document 1 disclose a method of determining a die configuration by estimating springback as described above. In particular, residual stress in a steel plate pressed in a die at the press bottom dead center is analyzed by a finite element method, and a die having a configuration of deformation (i.e., spring forward) caused by a residual stress toward the direction opposite to the aforementioned residual stress is numerically analyzed. In this manner, a die configuration which addresses the problem of springback can be obtained easily.
However, it is very difficult to design a die through numerical analysis taking the problem of the springback into consideration in a complete manner, because it is a nonlinear problem. The methods in the foregoing documents are proposed only to obtain a simple die which is designed taking a consideration of springback by the finite element method. The documents therefore suggest no countermeasures against a product obtained by press-forming in a die that is outside of the tolerance for springback, which is a phenomenon that is difficult to analyze numerically.
If a formed product which satisfies the tolerance value for the springback cannot be obtained using a die designed considering the springback problem, countermeasures to be undertaken must be determined by experienced technical personnel. Accordingly, there is a need to produce an actual die and to repeatedly modify the die configuration while pressing steel plates in the die.
Another approach to reduce the springback is to modify configurations of steel plates or formed products, not the configuration of the die, to remove residual stress. An exemplary modification method is to provide an opening or a slit in the formed product at an area where springback is occurring.
This approach can reduce residual stress which may otherwise cause springback by undertaking a countermeasure against areas where springback is occurring. However, since cutting or punching may decrease rigidity of the product itself, only slight residual stress tends to cause great springback. For this reason, this approach fails to completely eliminate the springback problem. In addition, such an approach needs tests with an actual test die and a steel plate, which increases man-hours and costs at the design stage.
Patent Documents 2-5 also disclose simulations by the finite element method. The methods disclosed in Patent Documents 2-4 employ partial stress release and modification. In Patent Document 2, however, evaluation is only made with respect to an amount of angle variations, i.e., torsion, before and after springback occurs in parts and thus factors that cause deformation other than torsion are outside of the discussion. In Patent Document 2, all the stress components at release positions during stress release are set to 0. If deformation is large, linear approximation performed with respect to stress gradients produces larger inconsistency between the linear approximation and actual nonlinear transition.